Hey, I bought a "non working" NTSC/JP Mega CD Model 1 from evilbay, and set about fixing what i thought was going to be either a blown fuse, or a blown fuse and a re-cap. Well, the fuse had gone, so i replaced it with the correct type and i was rewarded with just the Mega CD background, no sound or scaling logo, and just the GREEN light on the front of the unit... Mmmm. Ok so i though RE-CAP. so i set about removing the old nasty leaky caps, and replacing them with new ones.. so far so good. Re assembled and switched on to get.. The same??? Just the static background image etc.. So i thought it must be one or more of the actual chips beyond repair, so.. unlucky, tough shit, my fault.. Last ditch i put the NTSC/JP main board in my fully working PAL/EU Mega cd and it works.. both lights, music and animated logo (very distorted though). So it must be one of the other boards in the NTSC/JP Mega CD? NO... When i put the PAL/EU main board in the NTSC/JP, it works great.. perfect.. no issues. Anyone with a little more knowledge than me got a clue? Im Stumped...
My assumption would be that the connections to the secondary boards were a little loose. Have you tried switching the boards back again to their original machines?
So am I, it's quite puzzling indeed. I'll try some wild-guessing here based on the assumption that the scaler is having issues due to low or noisy voltage. Check for 5V on all electrolytic caps. If you find just a couple discrepancies, verify the tracks around them and redo the solder on nearby vias. If they're more or less low in an uniform manner, replace the ribbon that goes to CN3. If the voltages are still wrong after that, the power supply board is probably bad. Also, there's a very far-fetched possibility that the BIOS EPROM is losing its bit 'charges' and returning different values under slightly different supply voltages (it's rare but it does happen.) Question, can you post a pic of the distorted graphics? It may help pinpointing the failure point, hopefully. If all that fails I will be utterly baffled and ill-humored for the rest of the week. :nightmare:
The BIOS is not an EPROM, it is a mask rom. Very different technologies. That said this is common with damages traces but it can easily be capacitor related.
That's the crazy part, Sega used regular EPROMs on some model 1 SCD's, I took a pic of one of my boards: The other boards I have here were fitted with regular Sega mask ROMs. A rushed move to get them SCD's to the market quick perhaps?
Thanks guys, ill get a screen shot tomorrow, but the background image i.e. still image is fine, its the rotating and scrolling Mega CD logo that is distorted and blocky.. Here is a shot of what the caps looked like. After removing all the caps and cleaning the pads back to a nice silver solder, the new ones went on great.
Shows what I know. Possible some bit rot has occurred given the age of the hardware and the status of being an EPROM. I'd find it more likely that the capacitors took out a trace or 5.
Easily erased with UV yes but anything that is reprogrammable (and probably OTP items for that matter that aren't mask rom) can suffer bit rot. Plenty of Atari 2600 prototyes on EPROM and starting to fall apart these days.
The gates lose power over 10-20 years and bits start flipping back to their natural state of 1 (programming provides power that flips them to a 0, when this dissipates, its back to a 1) Reprogramming the chip should fix it, no need for a replacement.
This is the screen i get with the NTSC/JP main board in my PAL machine, all other components are from the Fully Functioning PAL Mega CD.. The PAL main board placed in the NTSC/JP Mega cd works flawlessly... But NTSC/JP main board back in the NTSC/JP Mega even after re-cap, is just black screen of death.. Ive given up for now, and have bought another PAL version to RAID for parts.. Thanks for all your suggestions, i'm gonna de-solder the PAL rom from the board and add a holder, then swap the eprom... :excitement:
Cde, I wanna change my statement, some low address lines from the QFP chip to the two DRAMs (near C28 and C31 on your picture, those two look pretty ruined BTW) are interrupted. Either the tracks themselves have been corroded or the vias, it's visible on the pic that there's some cap poop on the DRAMs' pads and vias. I wouldn't worry about the BIOS right now, cleaning up and resoldering that area should suffice. What we can infer from the picture is: 1) The connection between the SCD and Genny boards is okay, everything is in synch, main 68K talking to sub 68K, looks peachy. 2) Writes to the SCD's DRAM (is that the one Sega calls 'Work RAM'? I can't recall) are 'stuttering' every 8 pixels or so, that's something like A3,A4, or even A5 not being properly asserted. Ape: You think it's weird for them to have used EPROMs? What about that crazy daughterboard thingy? I've never seen it before! I wonder what that does? It even has an 'official' ID number. Druid: I'll attempt to take more detailed pictures of the oddball Sega hardware I have and post them here as soon as I can. Dunno why I didn't do that before. *facepalm*
WORD RAM (the two large ZIP package chips close to the LEDs at the front) have broken traces from it to he ASIC. Find where the broken traces are, patch them and your MEGA-CD will be good as new. WORD RAM is a pair of 16 bit DUAL PORTED DRAM chips which are used to allow for efficient data transfer with the two CPUs (so they don't need to stop their tasks during a transfer). And obviously it's used to send the logo data back from the scaler chip so it can be displayed by the VDP on the Mega Drive...
lol you did not write anything wrong. There's a WORK RAM too. WORD RAM is refered to in that way, because it's a programmable (programmable in the way the memory is placed on the system bus) set of memory (2 64KB chips by 16 bit) which is used to exchange *WORDS* of data from one to the other CPU. Hence the WORD RAM name. http://en.wikibooks.org/wiki/Genesis_Programming
